Group B Streptococcus (GBS) is the primary cause of invasive infections (e.g., bacteremia and meningitis) in newborns and pregnant women and an increasingly important cause of disease in the elderly. While a lot of attention has recently been focused on the discovery of GBS virulence genes, little is known about the contributions and interactions between them towards the pathogenicity of GBS. We propose a detailed molecular epidemiologic study of invasive and maternal colonizing isolates obtained from prospective population-based surveillance and two pregnancy- related cohort studies conducted in the Province of Alberta, Canada (population approximately 2.8 million) over an eight-year period. Specific Aim1: To characterize the clonal relationships and multiple virulence gene profiles of invasive GBS in a population based study. Specific Aim 2: To compare differences in the virulence gene profiles between colonizing vaginal-rectal derived GBS isolates and invasive isolates. Specific Aim 3: To investigate the impact of the delivery process and antibiotic treatment on the clonal composition and antigenic diversity of GBS colonizing the vaginal-rectal tract. Our ability to use GBS isolates from a population based study to identify specific virulence genes, virulence gene profiles, and certain clonal types that are associated with both colonization and clinical disease would greatly improve our understanding of the pathogenesis of invasive GBS disease. It will also aid in epidemiological studies to establish relationships between strains and determine what role if any specific phenotypes should play in vaccine development. In addition, the assessment of colonizing GBS strains will provide fundamental insights into the nature of GBS colonization, which is the critical first step preceding neonatal transmission, infection and disease.